This invention relates to electrostatography, and, more particularly, it relates to novel pressure-fixable type electroscopic marking particles, or, as commonly called, pressure-fixable toner material, and to the method of preparing same.
In electrostatography it is well known to tone electrostatic latent images contained on the surface of a recording member, such as a photoconductor or dielectric material, with dry toners which form pressure-fixable image deposits. Such image or toner deposits are, in certain instances, pressure fixed directly onto the recording member whereas in other instances the toner deposits are formed on the surface of a reusable photoconductor, such as, for instance, selenium, and transferred therefrom, usually by electrostatic methods, onto a receptor such as paper and affixed thereto by pressure. The pressure-fixing process involves normally passing the receptor such as the paper sheet containing the unfixed toner deposits thereon through the nip of a pair of rotating pressure rollers.
Prior art pressure-fixable toners are of the so-called two components or single component type. Two component toners are admixed with a carrier powder to form a developer mixture which is applied to the latent image-bearing recording member surface, as by a magnetic brush or by cascading, as is well known in the art; the toner is usually treated with a polarity control or charge control or tribocharging agent so that by admixture with the appropriate carrier powder and consequent tribocharging, the desired polarity will be conferred onto the toner, for instance, negative polarity if it is desired to develop a latent image formed by positive charges on a recording member, such as a selenium photoconductor.
Single component pressure-fixing toners usually contain a magnetic pigment such as magnetite or ferrite. The applicator for single component toners normally comprises a rotating multiple magnet roll contained within a stationary or rotating non-magnetic sleeve to the surface of which the toner is attracted, forming a magnetic brush thereon; such magnetic brush is contacted with the surface of the latent image-bearing recording member and development occurs by toner attraction to the charge-holding latent image areas. Here again it is necessary that the toner be of the correct polarity or be correctly tribocharged in relation to the polarity of the latent images on the recording member, for instance, to be of negative polarity, if it is desired to develop a latent image formed by positive charges on a recording member, such as a selenium photoconductor. Although correct polarity or charging or tribocharging of single component toners can be attained by such means as applying a bias voltage between the magnet roll applicator or other parts of the toning assembly and the conductive backing of the recording member or selecting of metals or materials for parts in contact with the toner in the toning assembly so as to be appropriately located in the tribo series, usually the single component toners are treated with polarity or charge controlling or tribocharging agents to confer the desired polarity onto such toner material.
Prior art methods of preparing pressure-fixable two component and single component toners are disclosed, for instance, in U.S. Pat. Nos. 3,804,764 of Strella et al. and 3,925,219 of Strong, respectively.
A typical prior art method of preparing two component toners incorporating a charge or polarity controlling agent is disclosed in U.S. Pat. No. 4,206,064 of Kiuchi et al.
Typical methods of preparing prior art pressure-fixable single component toners with charge or polarity controlling agents are disclosed, for instance, in U.S. Pat. Nos. 4,311,779 and 4,362,803 of Miyakawa et al.
From the above references it will be seen that prior art pressure-fixable toners are prepared, in essence, by hot blending and kneading a pressure-fixable binder material such as polyethylene wax or a polymer, such as polystyrene and the like, with a colored or magnetic pigment and a charge control agent; upon cooling, the blend is crushed and pulverized, typically by jet milling, following which the powder is classified to the required toner particle size range, normally about 5 to 30 microns. It will also be seen that prior art charge or polarity control agents comprise, in general, oil soluble dyes, alcohol soluble dyes, dyes with metal complexes, and other compounds containing metals or metal complexes.
Prior art pressure-fixable toners have certain disadvantages. The polymers and waxes used in such toners as the pressure-fixable medium are characterized by having a hardness within a certain range and a softening point within a certain temperature range, hardness and softening point being in many materials closely related to each other. Materials of lesser hardness and softening point of lower temperature can be fixed at lower pressure. However, they present storage and handling problems in that toner particles comprising such materials, at slightly elevated temperatures, tend to adhere to each other and form agglomerates. This impairs the flow properties of the toner and results in uneven image formation on the recording member. Materials having a hardness and softening point above said range can be fixed at extremely high pressures only, which is unacceptable. Even those materials which are within said range, that is, where their hardness and softening point temperature are not likely to cause interparticulate adhesion and agglomeration, require high pressures to achieve an adequate degree of fixing. This results, first, in the need for very robust and consequently heavy and expensive pressure roller system; second, in limiting the speed at which paper receptor can be passed through the pressure rollers without creasing or mistracking due to the high pressure at which it is transported therethrough; and, third, in what is called calendaring the receptor paper, that is, imparting undesirable gloss to its surface. A further disadvantage of such prior art pressure-fixing toners is that they tend to contaminate the surface of reusable photoconductive or dielectric recording members by formation thereon of a thin polymer or wax film as such toner material is repeatedly applied to the recording member surface. This results in the need to clean the recording member surface at relatively frequent intervals, which is expensive and, in addition, shortens the useful life.
Another disadvantage of such prior art pressure-fixable toners relates to the types of charge or polarity control agents used therein and their method of incorporation. As disclosed in the above references, the charge or polarity control agents which confer negative polarity comprise oil soluble dyes, alcohol soluble metal containing dyes, compounds with metal complexes and the like. Such materials are dispersed in the wax or polymeric pressure-fixable binder during hot blending, and, consequently, they are distributed throughout the resulting toner particles rather than being only at the particle surface, which is the effective location for charge or polarity control. Those minute charge or polarity control particles which are at or on the toner particle surface are not firmly bonded thereto and can be dislodged during agitation in the toning unit or during repeated contact with the recording member surface, thereby causing contamination thereof, particularly since such charge or polarity control materials are chemically of non-inert nature.
The pressure required to fix the above-referred-to prior art toners is generally in the range of 200 to 300 pounds per lineal inch. Various methods have been recently proposed for the preparation of toners which can be fixed at lower pressures. An effective method which has been proposed consists in making toner particles which are porous or are in the form of aggregates, that is to say, toner particles containing discontinuities which upon application of relatively low pressure cause firstly disintegration of the particle and then fixing.
Toner particles containing discontinuities and fixable at low pressures in the range of about 76 to 150 pounds per lineal inch are disclosed in U.S. Pat. No. 4,254,201 of Sawai et al., incorporated herein by reference. In this case the toner particles are, in essence, porous aggregates where each aggregate consists of a multiplicity of granules comprising encapsulated pressure sensitive adhesive material. The encapsulating material causes formation of the aggregate by adhering granules to each other. The colored or magnetic pigment is contained within the aggregate in the interstices between the granules and on the surfaces of the encapsulated granules. Upon application of pressure, the porous aggregate disintegrates or deforms, and then the pressure sensitive adhesive material is released from the individual encapsulated granules to bond the virtually free pigment particles onto the receiving surface.
Toner particles containing discontinuities and fixable at even lower pressures, in the range of about 55 to 76 pounds per lineal inch, are disclosed in U.S. Pat. No. 4,379,825 of Mitushashi. In this case the toner is prepared by mixing, and then hot kneading, in essence, a pressure-fixable binder, a colored or magnetic pigment, and a so-called elimination compound which remains in solid form, as it does not melt at the temperature at which said binder softens or melts for hot kneading. Upon cooling, the mixture is crushed by a hammer mill and then pulverized in a jet mill. The powder is then immersed in a solvent which dissolves and removes the elimination compound, thus forming a porous particle structure. The porous particles are then washed, filtered, dried, and classified to the required size range. The pores or discontinuities in the particle render it brittle or fragile, and, therefore, it can be more readily crushed and pressure-fixed than particles having a solid or homogeneous structure.
The porous particles are of irregular shape and of irregular structure in both surface and interior. The patent states, in column 2, lines 11-14, that true spherical shape particles are unsuitable for pressure-fixing.
While the above referred to toner particles containing discontinuities can be fixed at low pressures, they have certain disadvantages. The porous aggregate particles of U.S. Pat. No. 4,254,201 of Sawai et al. contain, of necessity, free colored or magnetic pigment on their surfaces, which makes these toner particles prone to cause contamination of the recording member, toning unit, and copier interior. The porous particles of U.S. Pat. No. 4,379,825 of Mitushashi are produced by a complicated process involving many steps, and, in addition, the process is wasteful because of the need to dissolve and then discard the elimination compound and the solvent therefor. Furthermore, Mitushashi states that the magnetite content of his toners may vary between 15 and 70 percent by weight, his preferred proportion being 20 to 50 percent by weight. His examples contain between 26 and 46.5 percent magnetite. Most commercial toners contain between about 55 to 65 percent by weight magnetite, and toners containing more than 55 percent by weight magnetite, made in accordance with these teachings, are extremely friable and tend to break up on the toning unit, giving rise to the production of foggy and low density copy, as well as dusting. In addition, both above-referred-to types of toner particles have the common disadvantage in that the charge or polarity control agents have to be included within the body of the toner material in the same manner as in prior art pressure-fixing toners as described previously.